pyridoxal 5'-phosphate (PLP) and is used by over 60 enzymes. Glycogen phosphorylase, an enzyme involved in the breakdown of muscle glycogen, requires PLP as a coenzyme. Moreover, glycogen phosphorylase may serve as a reservoir for vitamin B6 storage (Merrill and Burnham, 1990) and release PLP into the circulation for use by other tissues.
Vitamin B6 status in blood samples can be assessed in several ways (Driskell, 1984). The method of choice is to assess levels of PLP, the most active form of vitamin B6 (Driskell, 1984). Chronic exercise does not appear to result in a vitamin B6 deficiency. Although biochemical deficiencies for vitamin B6 were found in 17 to 35 percent of male college athletes, similar percentages were found for the control group (Guilland et al., 1989). However, the athletes had a greater intake of vitamin B6 compared with the control subjects. Adequate vitamin B6 levels were found from assessments of blood samples of other groups of athletes (Cohen et al., 1985; Weight et al., 1988).
Although it seems that vitamin B6 status is not altered by chronic exercise, some studies have shown that acute exercise can alter the blood levels. Leklem and Shultz (1983) found that a 4500-m run substantially increased the blood levels of PLP in trained adolescent males. Hatcher et al. (1982) and Manore and Leklem (1988) reported an increase in blood levels of PLP after a 50-minute and after a 20-minute cycling exercise. PLP levels returned to baseline values after only 30 minutes rest (Manore and Leklem, 1988). It was suggested (Leklem and Shultz, 1983; Manore and Leklem, 1988) that PLP may be released from muscle glycogen phosphorylase during exercise so that PLP could be used as a cofactor for gluconeogenesis elsewhere in the body.
Holmann et al. (1991) also found that prolonged treadmill running (2 hours at 60 to 65 percent of ) resulted in significant increases in blood levels of PLP that were independent of changes in plasma volume, blood glucose, blood free fatty acid levels, and blood enzyme levels. The authors suggested that the increase in plasma PLP could be due to a release of vitamin B6 from the liver to be used in skeletal muscle to fully saturate glycogen phosphorylase or be used for other critical PLP-dependent reactions (for example, aminotransaminase reactions).
Another study found that 4-pyridoxic acid excretion in urine was significantly lower in trained athletes compared with controls after a vitamin B6 challenge (Dreon and Butterfield, 1986). The authors suggested that these results reflect a greater storage capacity in athletes so that 4-pyridoxic acid could be available for redistribution with increased need.
Supplementation with vitamin B6 does not appear to enhance performance. Lawrence et al. (1975a) examined swimming performance of trained swimmers who ingested 51 mg of pyridoxine hydrochloride or a placebo